Capacitor with hafnium dioxide dielectric

ABSTRACT

HAFNIUM FILM CAPACITORS ARE PREPARED BY REACTIVELY SPUTTERING HAFNIUM DIOXIDE UPON AN ALUMINUM ELECTRODE, SUBJECTING THE RESULTANT ASSEMBLY TO ELECTROLYTIC ANODIZATION AND DEPOSITING A COUNTERELECTRODE THEREON.

.Fan. 5, 1971 GERSTENBERG ET AL I 355%@ CAPACITOR WITH HAFNIUM DIOXIDE DIELECTRTC Original Filed Feb. 26, 1968 FlGgi 0. GERS TENBERG /N/E/VTORSE AT TONA/E V United States Patent O 1nt. Cl. H01g 1/0] U.S. Cl. 317--258 2 Claims ABSTRACT OF THE DISCLOSURE Hafnium film capacitors are prepared by reactively sputtering hafnium dioxide upon an aluminum electrode, subjecting the resultant assem'bly to electrolytic anodization and depositing a counterelectrode thereon.

This is a division of copending application, Ser. No. 708,339, filed Feb. 26, 1968.

The present invention relates to thin film capacitors including a layer of aluminum as one of the electrodes, hafnium dioxide as the dielectric, and an electrically conductive counterelectrode.

In recent years there has been widespread interest in the electronics industry in a class of capacitors commonly referred to as printed capacitors. These structures are typically constructed by depositing a layer of a filmforming metal upon a substrate, anodizng the deposited layer to form an oxide film and finally depositing a counterelectrode upon the anodized film. The resultant devices are found to be 'polar in nature and when initally produced represented the first such device in which a semiconductive layer of manganese dioxide was eliminated, such having been a requirement in solid electrolytic capacitors produced theretofore.

At that juncture in the chronological history of capacitor development, it Was believed that the printed capacitor represented the ultimate objective in the development of capacitors employing an electrode comprising a film-forming metal. Although this type of capacitor is eminently suited for use in printed circuitry, its importance in this use has resulted in a continuing eiort to improve it. Accordingly, workers in the art have long sought to enhance the electrical characteristics of such devices.

In accordance with the present invention, a thin film capacitor evidencing superior dielectric strength, humidity sensitivity, and dissipation factor, as Compared with prior art structures is described. The inventive device is obtained by reactively sputtering a hafnium dioxide dielectric upon an aluminum electrode and subjecting the resultant assembly to electrolytic anodization.

The invention will be more readily understood by reference to the following detailed description taken in conjunction with the accompanying drawing, wherein:

FIG. 1 is a plan View of a substrate With a pattern of aluminum deposited therein;

FIG. 2 is a plan view of the body of FIG. 1 after deposition thereon of a reactively sputtered layer of hafnium dioxde;

FIG. 2A is a cross-sectional view of the body of FIG. 2 showing pinhole defects in the hafnium dioxide layer FIG. 3 is a cross-sectional view of the body of FIG. 2A after electrolytic anodization; and

FIG. 4 is a plan view of the body of FIG. 3 after the deposition of a counterelectrode thereon.

With further reference now to FIG. 1, there is shown a substrate 11 having a pattern of aluminum 12 deposited thereon. The fabrication procedure contemplates the use of a substrate material which is able to withstand temperatures to which they may be subjected during the deposition stages of the processing. Glasses and glazed ceramics are particularly suitable in this use.

Initially, the substrate member is cleansed by conventional technques well known to those skilled in the art. Following the cleaning step, a layer of aluminum 12 is deposited by conventional condensation techniques, as, for example, 'vacuum evaporation, cathodic sputtering, and so forth, as described by L. Holland in Vacuum Deposition in Thin Flms, J. Wiley & Sons, 1956. From a theoretical standpoint, an anodizable metal such as tantalum, hafnium, nio'bium, and so forth, should be suitable for use in the described structure. However, it has been determined that the electrical characteristics of these materials are subject to degradation after deposition thereon of reactively sputtered hafnium dioxide, such being attributed to the tendency of oxygen to diffuse from the hafniurn dioxide film into the underlying metal anode layer during the reactive sputtering stage, thereby resulting in an oxygen deficient hafnium dioxide dielectric. Therefore, aluminum is chosen as the anodizable metal since no difiusion of oxygen is found to occur during the deposition stage.

For the purposes of the present invention, the thckness of the aluminum anode ranges from 1000-10,000 A. The use of aluminum layers thinner than 1,000 A. fails to result in fabricaton of an operative device whereas layers appreciable beyond 10,000 A. in thickness adversely afiect device properties due to mechanical instability.

Following the deposition of the aluminum anode, the assembly is placed in a sputtering apparatus including a hafnium cathode or an aluminum disk covered with hafnium, for example, in the form of a foil. The apparatus is next evacuated and oxygen is admitted at a dynamic pressure and, after attaining equili'brium, argon is admitted. The extent of the vacuum is dependent upon consideraton of several factors.

Increasing the inert gas pressure and thereby reducing the vacuum within the vacuum chamber increases the rate at which the hafnium being sputtered is removed from the cathode and, accordingly, increases the rate of deposition. The maximum pressure is usually dictated by power supply limitations since increasing the pressure also increases the current flow between the cathode and the anode in the sputtering cham ber. A practical upper limit in this respect is 20 microns of mercury for a sputtering voltage of 3000 volts, although it may be Varied depending on the size of the cathode, sputtering rate, and so forth. The ultimate maximum pressure is that at which the sputtering can be reasonably controlled within the prescribed tolerances. It follows from the discussion above that the minimum pressure is determined by employing the lowest deposition rate which can be economically tolerated.

After the requisite pressure is attained, the cathode which may be composed of hafnium or, alternatively, an aluminum disk covered with hafnium is made electrically negative with respect to the anode.

The minimum voltage necessary to produce sputtering is about 3000 volts. Increasing the potential difference between the anode and cathode has the same efiect as increasing the pressure, that of increasing both the rate of deposition and the current flow. Accordingly, the maxi mum voltage is dictated by consideraton of the same factors controlling the maximum pressure.

The spacing between anode and cathode is not critical. However, the minimum separation is that required to produce a glow discharge which must be present for sputtering to occur. Many dark striations in the discharge are well known and have been given names as, for example, Crooke*s Dark Space. For the best eiciency during the sputtering step, the substrate should be positioned immediately without Crooke`s Dark Space on the side closest to the anode. Location of the substrate closer to the cathode results in a metal deposit of poorer quality. Locating the substrate further from the cathode results in impingement on the substrate by a smaller fraction of the total metal sputtered, thereby increasing the time necessary to produce a deposit of given thickness.

It should be noted that the location of Crooke`s Dark Space changes with variations in the pressure, it moving closer to the cathode with increasing pressure. As the substrate is moved closer to the cathode, it tends to act as an obstacle in the path of gas ions bombarding the cathode. Accordingly, the pressure should be maintained sufficiently low so that Crookes Dark Space is located beyond the point at which a substrate would cause shielding of a cathode. The balancing of these various factors of voltage, pressure and relative positions of the cathode and of the substrate to obtain a high quality deposit is well known in the sputtering art.

By employing a proper voltage, pressure, and spacing of the elements within the vacuum chamber, a layer of hafnium dioxide 13 (FIG. 2) is deposited in a configuration defined by a sputtering mask which conforms to the configuraton of the aluminum layer. Sputtering is conducted at oxygen partial pressures within the range of 10" -l0- torr. for a period of time calculated to produce the desired thickness.

With reference now to FIG. 2A, there is shown a cross-sectional View of the body of FIG. 1 after the deposition thereon of the reactively sputtered layer of hafnium dioxide 13. Pnhole defects 14 and 15 in the hafnium dioxide layer 13 adversely afect the dielectric properties of layer 13 and dictate a further anodization procedure whereby the aluminum layer 12 underlying the hafnium dioxide layer will be oxidized at the sites of the pinholes. Anodization is efected in an appropriate electrolyte. The voltage at which the anodizing is conducted is primarily determined by the thickness of the hafnium dioxide layer. The usual procedure to be followed in eecting the anodization procedure is similar to conventional anodizing processes in which a low voltage is applied initally, and the voltage is then increased so as to maintain a constant anodizing current. Examples of low conductivity electrolytes suitable for this purpose are aqueous solutions of ammonium pentaborate, oxalic acid, citric acid, tartaric acid, and so forth. A cross-sectional View of the body of FIG. 2A after anodization is aluminum and gold being Conveniently used in conjunction with this technique, the evaporated layer being restricted by means of a mask. FIG. 4 is a plan view of the body of FIG. 3 after the deposition thereon of a counterelectrode 18. Since the counterelectrode must conduct all of the current which passes through the capacitor, its electrical resistance is desirably low. The minimum thickness is approximately 500 A., a preferred range being from 1000-2000 A.

An example of the present invention is included merely to aid in the understanding of the invention.

It will be understood that variations may be made by one skilled in the art without departing from the spirit and scope of the invention.

EXAMPLE A glass plate approximately 1" in width and 3" in length was cleaned ultrasonically by conventional techniques. Thereafter, a layer of aluminum 5000 A. in thickness was deposited upon the substrate member by vacuum evaporation techniques. Next, the resultant assembly was placed in a sputtering apparatus including a hafnium cathode, the apparatus evacuated and oxygen admitted at a dynamic pressure. After obtaining equilibrium, argon was admitted and sputtering effected by impressing a difference of potential of 4000 volts across the anode and cathode, the oxygen partial pressure in the system being maintained at approximately 2.0 x 10' torr. Sputtering was conducted for 20 minutes, so resulting in the deposi tion of a hafnium dioxide layer 2000 A. in thickness upon the aluminum.

Next, anodization of the resultant assembly was effected electrolytically at 150 volts at a current density of 0.1 milliamperes per square centimeter in a electrolyte comprising a 30 percent ammonium pentaborate solution in ethylene glycol. Anodization Was conducted for 30 minutes. The capacitor was completed by the deposition thereon of a gold counterelectrode by vacuum deposition.

For comparative purposes, capacitors produced in accordance with the present invention were compared with anodized tantalum and hafnium capacitors and with reactively sputtered hafnium dioxide on aluminum capacitors (non-anodized). The anodized devices were prepared by cathodically sputtered tantalum or hafnium films 4000 A. in thickness upon a substrate member and anodizing and completing the structures as described above. The reactively sputtered hafnium dioxide films were produced as described herein but were not subjected to the anodizing treatment. The results of the comparison are set forth in the table below:

TABLE .Anodized Anodized sputtered sputtered and ano- Property tantalum hafnium HFOz on Al dized HFOz on Al Breakdown voltage:

(a) forward 95-100 50-60 35-40 145-150 10-30 20-30 35-40 35-40 0. 004 0. 008-0. 02 O. 0015-0. 002 0. 001-0. 0015 (b) At 100 kHz 0. l-O. 2 0. 02-0. 04 0. 0015-0. 004 0. 001-0 004 Change in capaeitance on cycling from 0-87%, relativo humidity (percent) shown in FIG. 3. Shown in the figure are pinhole defects 14 and 15 including anodized aluminum layers 16 and 17 contained within the cavities of the detect areas 14 and 15. V

The last step in the fabrication of a capacitor in accordance with the invention is the application of a counterelectrode in contact with the hafnium dioxide film. Any suitable method for producing an electrically conductive layer on the surface of the dioxide layer is suitable provided such method does not mechanically or thermally disturb the dioxide layer. Vacuum evaporation has been found to be especially suited for producing counterelectrodes in accordance with this invention, metals such as 

